The present invention relates to a method and apparatus for measuring the surface contour of a test piece which is completely immersed in water or any other liquid which transmits ultrasonic waves.
Ultrasonic techniques have been used in the past for the non-destructive testing of a test piece or the like to determine the presence of flaws therein. These ultrasonic techniques measure the time it takes for an ultrasonic wave to travel from a transducer to the test piece and for the reflected wave to travel back to the transducer. Surface contour measurement using ultrasonics is possible because sound waves are reflected from the surface of the test piece. Thus, when measuring the surface contour of a test piece any surface portion which is curved or somehow misshaped will be detected, because the time necessary for the ultrasonic wave to travel from the transducer to the surface of the test piece and back will change.
One of the types of ultrasonic inspection involves immersion testing where the test piece is completely submerged in a tank containing a liquid, such as water, and acoustically coupled by the fluid to the transducer. The sound waves leave the transducer, travel through the water to the test piece. Echoes are returned to the transducer from the surface of the test piece, and the amount of time it takes the wave to leave and echo back to the transducer is measured.
This measured time is a function of both the distance the wave must travel and the velocity of sound in the medium through which the wave travels. Once the velocity of sound in the medium is known, the distance can be determined accurately by measuring the time of flight with a common oscilloscope or ultrasonic flaw detection equipment. With these ultrasonic techniques, the limiting factor is an accurate measurement of the velocity of sound in the transmitting medium. If the medium between the transducer and the test piece is liquid at a known constant temperature then the velocity of sound in the medium can easily be determined. Once the velocity of sound in the medium is known, the distance between the transducer and the surface of the test piece can also be easily determined.
However, testing a test piece which is at a temperature different from that of the surrounding liquid medium can cause inaccurate measurement. For example, when the surface of the test piece is at a higher temperature than the surrounding liquid, where such liquid is for example water, a path of water with an unknown temperature gradient is created between the transducer and the surface of the test piece. This makes it difficult to accurately determine the temperature of the water over the entire distance from the transducer to the surface of the test piece. Since the velocity of sound in water depends significantly on its temperature, as shown in FIG. 1, and since it is necessary that the changes in of the surface contour be measured within .+-.0.005 inch, the use of the conventional ultrasonic distance measuring systems, as described above, cannot provide accurate measurements, when testing the surface contour of a test piece having a temperature higher than the temperature of the water in the tank.
Accordingly, the method of the present invention was devised which enables testing the surface of a test piece for its contour (i.e., to determine if it is bowed), which test piece has a surface temperature different from that of the surrounding liquid medium, especially test pieces having a surface at an elevated temperature. Thus, the present invention can be used to measure the surface contour or bow of radioactive fuel rods, etc.